the degree. From then on, the twenty-three-year-old physicist could call himself Dr. Stephen Hawking.
5
FROM BLACK HOLES TO THE BIG BANG
I n the early 1960s, astronomers already knew that any star which contains more than about three times as much matter as our Sun ought to end its life by collapsing inward to form what is now known as a black hole. More than twenty years previously, researchers had used Einsteinâs equations of general relativity to calculate thatsuch an object would bend spacetime completely around upon itself, cutting the central mass off from the rest of the Universe. Light rays passing near such an object would be deflected so much that even photons would orbit around the central âstarâ in closed loops and could never escape into the Universe outside. Obviously, since it could emit no light, such an object would be black, which is why the American relativist John Archibald Wheeler dubbed them âblack holesâ in 1969.
But although it was well known that the general theory made this prediction, at the time Hawking was completing his undergraduate studies and moving on to research, no one took the notion of black holes seriously. The reason is that there are very many known stars that have more than three times the mass of our Sun. They do not collapse because nuclear reactions going on inside the stars make them hot. The heat creates an outward pressure that holds the star up against the pull of gravity. Astronomers knew that when such stars run out of nuclear âfuel,â they explode, blasting away their outer layers into space. As recently as fifty years ago, astronomers assumed that such an explosion would always blow away so much matter that the core left behind would have less than three times the mass of our Sunâor, perhaps, that some as-yet undiscovered pressure would come into play as the remnant of star stuff began to shrink.
This prejudice was reinforced by the fact that astronomers had indeed discovered many old, dead stars. These stellar cinders all had a bit less than the mass of our Sun, but that mass was compressed into a volume only about as big as that of the Earth. Such planet-sized stars are known as white dwarfs. They are held up against the inward pull of gravity by the pressure of the electrons associated with the atoms of which they are made, acting like a kind of electron gas. A white dwarf is sodense that each cubic centimeter of the star contains a million grams of material. Before 1967, these were the densest known objects in the Universe.
But although astronomers did not seriously believe that anything denser than a white dwarf could exist, a few mathematicians enjoyed playing with Einsteinâs equations to work out what would happen to matter if it were squeezed to still greater densities. The equations said that if three times as much matter as our Sun contains were squeezed until it occupied a spherical region with a radius of just under 9 kilometers, spacetime in its vicinity would be so distorted that not even light could escape. Because nothing can travel faster than light, this meant that nothing at all could ever escape from such an object, which the mathematicians sometimes referred to as a collapsar (from âcollapsed starâ). It would have become the ultimate bottomless pit into which anything could fall but from which nothing could ever emerge. And the density inside the collapsar would be greater than the density of the nucleus of an atom; this, theorists of the time thought, was clearly impossible.
In fact, they did consider (but not too seriously) the possibility of stars as dense as the nucleus of an atom. By the 1930s, physicists knew that the nucleus of an atom is made of closely packed particles called protons and neutrons. The protons each carry one unit of positive charge; the neutrons, as their name suggests, are electrically neutral, but each has about the same mass as a proton. In everyday atoms, like the
5
FROM BLACK HOLES TO THE BIG BANG
I n the early 1960s, astronomers already knew that any star which contains more than about three times as much matter as our Sun ought to end its life by collapsing inward to form what is now known as a black hole. More than twenty years previously, researchers had used Einsteinâs equations of general relativity to calculate thatsuch an object would bend spacetime completely around upon itself, cutting the central mass off from the rest of the Universe. Light rays passing near such an object would be deflected so much that even photons would orbit around the central âstarâ in closed loops and could never escape into the Universe outside. Obviously, since it could emit no light, such an object would be black, which is why the American relativist John Archibald Wheeler dubbed them âblack holesâ in 1969.
But although it was well known that the general theory made this prediction, at the time Hawking was completing his undergraduate studies and moving on to research, no one took the notion of black holes seriously. The reason is that there are very many known stars that have more than three times the mass of our Sun. They do not collapse because nuclear reactions going on inside the stars make them hot. The heat creates an outward pressure that holds the star up against the pull of gravity. Astronomers knew that when such stars run out of nuclear âfuel,â they explode, blasting away their outer layers into space. As recently as fifty years ago, astronomers assumed that such an explosion would always blow away so much matter that the core left behind would have less than three times the mass of our Sunâor, perhaps, that some as-yet undiscovered pressure would come into play as the remnant of star stuff began to shrink.
This prejudice was reinforced by the fact that astronomers had indeed discovered many old, dead stars. These stellar cinders all had a bit less than the mass of our Sun, but that mass was compressed into a volume only about as big as that of the Earth. Such planet-sized stars are known as white dwarfs. They are held up against the inward pull of gravity by the pressure of the electrons associated with the atoms of which they are made, acting like a kind of electron gas. A white dwarf is sodense that each cubic centimeter of the star contains a million grams of material. Before 1967, these were the densest known objects in the Universe.
But although astronomers did not seriously believe that anything denser than a white dwarf could exist, a few mathematicians enjoyed playing with Einsteinâs equations to work out what would happen to matter if it were squeezed to still greater densities. The equations said that if three times as much matter as our Sun contains were squeezed until it occupied a spherical region with a radius of just under 9 kilometers, spacetime in its vicinity would be so distorted that not even light could escape. Because nothing can travel faster than light, this meant that nothing at all could ever escape from such an object, which the mathematicians sometimes referred to as a collapsar (from âcollapsed starâ). It would have become the ultimate bottomless pit into which anything could fall but from which nothing could ever emerge. And the density inside the collapsar would be greater than the density of the nucleus of an atom; this, theorists of the time thought, was clearly impossible.
In fact, they did consider (but not too seriously) the possibility of stars as dense as the nucleus of an atom. By the 1930s, physicists knew that the nucleus of an atom is made of closely packed particles called protons and neutrons. The protons each carry one unit of positive charge; the neutrons, as their name suggests, are electrically neutral, but each has about the same mass as a proton. In everyday atoms, like the
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